In a conventional mouth cleaning device, an electric toothbrush or an ion toothbrush is known. The conventional mouth cleaning device removes plaque from teeth by flowing a minute current in a mouth, thereby enhancing the cleaning effect of brushing, metabolism of oral tissues or blood flow. For example, Japanese Patent No. 2560162 describes therein a toothbrush including a head portion having bristles and a handle portion having a battery. An electrode connected with one pole of the battery is arranged on a part of a surface of the handle portion, whereas an electrode connected with the other pole of the battery is arranged on the head portion. When a user inserts the head portion into a mouth while holding the handle portion, a minute current flows through the body from the hand holding the handle portion to the contact portions between the head portion and teeth or gums inside the mouth.
Further, to avoid an influence of a resistance variation of the human body upon a current, Japanese Patent Laid-open Application No. 2005-192578 (Reference 2) discloses a mouth cleaning device having a configuration in which the two electrodes respectively connected to an anode and a cathode of the battery are both accommodated in the head portion.
Since, however, the battery available on the market to be used in this type of mouth cleaning device has a voltage ranging from about 1.5 to 3 V and an average resistance of the human body is 80 kΩ, it has been difficult to flow enough current to sufficiently obtain a desired effect.
Moreover, though the average resistance value of the human body is 80 kΩ, it actually fluctuates from about 40 k Ω to 120 kΩ. Therefore, when a battery having a voltage of 3 V is used, a stationary current flowing in the device is made to vary from about 25 μA to 75 μA, so that an achieved effect varies greatly depending on individuals using the device.
Further, if it is attempted to increase a voltage by, e.g., connecting a plurality of batteries in series, the size of the device increases, which makes it costly and difficult to handle the device when brushing teeth. Further, with the increase of the voltage, there occurs a safety issue because a great amount of current may flow into the human body depending on the individuals.
At an initial stage of an electric conduction to the human body, a surge current can occur, which can inflict problems. Referring to FIG. 4, there is provided an equivalent circuit of a biological tissue, wherein a resistance value of a resistor R1 connected to a capacitor C in parallel is much larger than a resistance value of a resistor R2 connected to the capacitor C in series. In this case, at an initial stage of the electric conduction, a great amount of current (surge current) is flown in a path including the capacitor C for a short period of time until the capacitor C is charged, which is instantaneous. Then, upon the completion of the charging of the capacitor C, a minute current flows in a path including the resistor R1 having the larger resistance value. FIG. 5 is a graph showing the surge current and a stationary current thereafter.
The level of current which a living creature can perceive when the current flows in the mouth was investigated through animal experimentation. Animal behaviors indicating their perception of the current were observed at a current level of about 300 μA. Further, in an experiment for the human body in which a conduction path was set up from a mouth to a hand, a stationary current was about 40 μA and a surge current was about 1 mA. If the stationary current value is raised, the surge current also increases. Therefore, irritation is likely to be inflicted upon the human body. As a result, there occurs a safety issue.
As for Reference 2, if the inside of the mouth is filled with saliva, the two electrodes provided in the head portion having the bristles would be short-circuited via the saliva. In such case, a current hardly flows in the teeth or the gums, so that a desired effect cannot be obtained.